Hpdc Magnesium Alloy

ABSTRACT

A magnesium-rare earth-yttrium-zinc alloy consists of 0.2-1.5% by weight zinc and rare earth(s) (RE) and yttrium in amounts which fall within a quadrangle defined by lines AB, BC, CD and DA wherein: A is 1.8% RE-0.05% Y, B is 1.0% RE-0.05% Y, C is 0.2% RE-0.8% Y, and D is 1.8% RE-0.8% Y.

FIELD OF THE INVENTION

The present invention relates to magnesium alloys and, more particularly, to magnesium alloys which can be cast by high pressure die casting (HPDC).

BACKGROUND TO THE INVENTION

With the increasing need to limit fuel consumption and reduce harmful emissions into the atmosphere, automobile manufacturers are seeking to develop more fuel efficient vehicles. Reducing the overall weight of the vehicles is a key to achieving this goal. Major contributors to the weight of any vehicle are the engine and other components of the power train. The most significant component of the engine is the cylinder block, which makes up 20-25% of the total engine weight. In the past significant weight savings were made by introducing aluminium alloy cylinder blocks to replace traditional grey iron blocks, and further weight reductions of the order of 40% could be achieved if a magnesium alloy that could withstand the temperatures and stresses generated during engine operation was used. Development of such an alloy, which combines the desired elevated temperature mechanical properties with a cost effective production process, is necessary before viable magnesium engine block manufacturing can be considered.

HPDC is a highly productive process for mass production of light alloy components. While the casting integrity of sand casting and low pressure/gravity permanent mould castings is generally higher than HPDC, HPDC is a less expensive technology for higher volume mass production. The most common magnesium based HPDC alloys are AM50 (95% Mg, 5% Al), AM60 (94% Mg, 6% Al) and AZ91 (90% Mg, 9% Al and 1% Zn). Unfortunately, none of these alloys are suitable for use at elevated temperatures.

HPDC is gaining popularity among automobile manufacturers in North America and is the predominant process used for casting aluminium alloy engine blocks in Europe and Asia. In recent years, the search for an elevated temperature magnesium alloy has focused primarily on the HPDC processing route and several alloys have been developed. HPDC is considered to be a good option for achieving high productivity rates and thus reducing the cost of manufacture.

U.S. Pat. No. 3,718,460 which claims a priority date of 4 Dec. 1967 relates to a magnesium-aluminium-silicon alloy which is “particularly adaptable to die casting”. The alloy consists “essentially of magnesium containing by weight from about 0.4 to 1.5 percent silicon, from about 3.5 to about 7 percent aluminium, up to about 1 percent manganese and up to about 2 percent zinc”. U.S. Pat. No. 3,718,460 makes no mention of yttrium.

PCT/GB96/00261 (WO 96/24701) which claims a priority date of 6 Feb. 1995 relates to a magnesium-zinc-rare earth (Mg—Zn-RE) alloy for HPDC in which the expression rare earth is specifically defined in terms of a range of elements but “is not intended to include elements such as yttrium.” Yttrium is thus specifically excluded as an alloy component of this HPDC alloy.

U.S. Pat. No. 6,322,644 which claims a priority date of 15 Dec. 1999 relates to a magnesium-based diecast alloy having improved elevated temperature performance which consists of 2-9% aluminium, 0.5-7% strontium, 0-0.6% manganese, 0-0.35% zinc and the balance magnesium. No mention of yttrium is made in U.S. Pat. No. 6,322,644.

Various magnesium based alloys which contain yttrium have been proposed over the years.

GB 1067915 which claims a priority date of 26 Oct. 1963 notes that “it has now been discovered that an addition of yttrium brings about a further refinement of the grain of a zirconium-containing magnesium alloy.” The patent is broadly directed to magnesium alloys containing 0.1-1% zirconium (Zr), 0.1-10% yttrium (Y), and up to 10% of at least one additional alloying element selected from beryllium (Be), lead (Pb), cadmium (Cd), calcium (Ca), cerium (Ce), copper (Cu), silver (Ag), thallium (Tl), thorium (Th), bismuth (Bi) and zinc (Zn).

The magnesium alloy ML10, developed in the former USSR, has been used for many years for cast parts intended for use in aircraft at temperatures up to 250° C. ML10 is a high strength Mg—Nd—Zn—Zr alloy. ML19 alloy is similarly based on the Mg—Nd—Zn—Zr system but additionally contains Y.

A paper by Mukhina et al entitled “Investigation of the Microstructure and Properties of Castable Neodymium and Yttrium-Bearing Magnesium Alloys at Elevated Temperatures” published in Science and Heat Treatment” Vol 39, 1997, indicates typical compositions (% by weight) of ML10 and ML19 alloys are:

ML10 ML19 Neodymium (Nd) 2.2-2.8 1.6-2.3 Yttrium (Y) Nil 1.4-2.2 Zirconium (Zr) 0.4-1.0 0.4-1.0 Zinc (Zn) 0.1-0.7 0.1-0.6 Magnesium (Mg) Balance Balance with impurity levels of:

Iron (Fe) <0.01 Silicon (Si) <0.03 Copper (Cu) <0.03 Nickel (Ni) <0.005 Aluminium (Al) <0.02 Beryllium (Be) <0.01

ML10 and ML19 are both sand casting alloys and neither has found commercial acceptance as a HPDC alloy.

GB 1378281 which claims a priority date of 14 Mar. 1973 “relates to magnesium-based light structural alloys, particularly those for the production of parts subject to heating in service.” The alloy contains 0.8-6.0% Y, 0.5-4.0% Nd, 0.1-2.2% Zn, 0.31-1.1% Zr, up to 0.05% Cu, up to 0.2% manganese (Mn) and the balance Mg. A related US patent, U.S. Pat. No. 4,116,731, claims an alloy of identical composition which is a “heat-treated and aged” alloy in which “no less than 50% of the total amount of neodymium and yttrium additions enters the solid solution after heat treatment” and the alloy, having been heat treated at approximately 535° C. for 4-8 hours, is cooled in air and then aged at approximately 200° C. for 12 hours.

U.S. Pat. No. 4,401,621 which claims a priority date of 25 May 1981 relates to magnesium alloys consisting of:

“(a) from 1.5 to 10% by weight of an yttrium component consisting of at least 60% by weight of yttrium and the balance, if any, of heavy rare earth metals, and

(b) from 1 to 6% by weight of a neodymium component consisting of at least 60% by weight of neodymium, not more than 25% by weight of lanthanum and substantially all the balance, if any, of praseodymium,

the remainder of the alloy consisting of magnesium.”

U.S. Pat. No. 6,767,506 which claims a priority date of 10 Jan. 2002 “relates to magnesium-based alloys suitable for applications at temperatures as high as 250-300° C.”. Alloys according to U.S. Pat. No. 6,767,506 contain 2.7-3.3% Nd, Y in amounts up to 2.6%, 0.2-0.8% Zr, 0.2-0.8% Zn, 0.03-0.25% Ca, 0-0.001% Be and at least 92% Mg. The alloys are said to be well adapted for sand casting, permanent mould casting and direct chill casting with subsequent extrusion and/or forging. There is no suggestion in U.S. Pat. No. 6,767,506 that the alloys are suitable for HPDC.

Mg-RE-Y alloys tend to be used as gravity and sand casting alloys which can be heat treated to achieve desired properties. They tend to have rather high additions of both RE and Y with the aim of having a phase at grain boundaries which is of the Mg-RE type and two precipitating phases, namely Mg₁₂Nd and Mg₂₄Y₅. Y has a high solubility in Mg even at room temperature and so high levels of Y are necessary to achieve any significant level of precipitation. As far as the present inventors are aware, no Y containing Mg-based alloy has found commercial acceptance as a HPDC alloy.

SUMMARY OF THE INVENTION

In a first aspect, the present invention provides a magnesium-rare earth-yttrium-zinc alloy consisting of:

rare earth(s) (RE) and yttrium (Y) in amounts which fall within a quadrangle defined by lines AB, BC, CD and DA wherein:

A is 1.8% RE-0.05% Y,

B is 1.0% RE-0.05% Y,

C is 0.2% RE-0.8% Y, and

D is 1.8% RE-0.8% Y;

0.2-1.5% zinc (Zn);

0-0.25% aluminium (Al);

0-0.2% zirconium (Zr);

0-0.3% manganese (Mn);

0-0.1% oxidation inhibiting element(s), and

the balance being magnesium (Mg) except for incidental impurities. Unless otherwise stated, all percentages in this document are % by weight.

Throughout this specification the expression “rare earth” is to be understood to mean any element or combination of elements with atomic numbers 57 to 71, ie. lanthanum (La) to lutetium (Lu).

The quadrangle defined by lines AB, BC, CD and DA is illustrated in FIG. 1 which is a plot of total rare earth content versus yttrium content.

Preferably, alloys of the present invention contain:

no more than 0.15% titanium,

no more than 0.15% hafnium,

no more than 0.1% copper,

no more than 0.1% nickel,

no more than 0.1% silicon,

no more than 0.1% silver,

no more than 0.1% thorium,

no more than 0.1% strontium, and

no more than 0.01% iron.

More preferably, alloys according to the present invention:

(a) contain less than 0.1% titanium, more preferably less than 0.05% titanium, more preferably less than 0.01% titanium, and most preferably substantially no titanium;

(b) contain less than 0.1% hafnium, more preferably less than 0.05% hafnium, more preferably less than 0.01% hafnium, and most preferably substantially no hafnium;

(c) contain less than 0.05% copper, more preferably less than 0.02% copper, more preferably less than 0.01% copper, and most preferably substantially no copper;

(d) contain less than 0.05% nickel, more preferably less than 0.02% nickel, more preferably less than 0.01% nickel, and most preferably substantially no nickel;

(e) contain less than 0.05% silicon, more preferably less than 0.02% silicon, more preferably less than 0.01% silicon, and most preferably substantially no silicon;

(f) contain less than 0.05% silver, more preferably less than 0.02% silver, more preferably less than 0.01% silver, and most preferably substantially no silver;

(g) contain less than 0.05% thorium, more preferably less than 0.02% thorium, more preferably less than 0.01% thorium, and most preferably substantially no thorium; and

(h) contain less than 0.05% strontium, more preferably less than 0.02% strontium, more preferably less than 0.01% strontium, and most preferably substantially no strontium.

Preferably, alloys of the present invention contain rare earth(s) and yttrium in amounts which fall within a quadrangle defined by lines EF, FG, GH and HE wherein:

E is 1.5% RE-0.3% Y,

F is 1.0% RE-0.3% Y,

G is 1.0% RE-0.8% Y, and

H is 1.5% RE-0.8% Y.

The quadrangle defined by lines EF, FG, GH and HE is illustrated in FIG. 1 which is a plot of total rare earth content versus yttrium content.

Preferably, alloys according to the present invention contain at least 96.7% magnesium, more preferably 97-98.5% magnesium, and most preferably about 98% magnesium.

Preferably, the rare earth component of alloys according to the first or second aspects of the present invention are selected from neodymium (Nd), cerium (Ce), lanthanum (La), or any mixture thereof.

Preferably, the neodymium content is greater than 0.2%, more preferably greater than 0.4%, more preferably 0.4-1.8% and most preferably 0.4-1.0%, although alloys of the present invention may contain no neodymium. The neodymium content may be derived from pure neodymium, neodymium contained within a mixture of rare earths such as a misch metal, or a combination thereof.

Preferably, the content of rare earth(s) other than neodymium is 0-1.6%, more preferably 0.5-1.0%, although alloys of the present invention may contain no rare earths other than neodymium. Preferably, any rare earth(s) other than neodymium are cerium, lanthanum, or a mixture thereof. Rare earth(s) other than neodymium may be derived from pure rare earths, a mixture of rare earths such as a misch metal or a combination thereof. Preferably, rare earths other than neodymium are derived from a cerium misch metal containing cerium, lanthanum, optionally neodymium, a modest amount of praseodymium (Pr) and trace amounts of other rare earths.

Without wishing to be bound by theory, the inclusion of yttrium is believed to be beneficial to melt protection, ductility and creep resistance.

Preferably, the zinc content is 0.2-0.7%, more preferably 0.3-0.5%, more preferably 0.4-0.6%.

Zirconium is an optional component of alloys of the present invention. Reduction in iron content can be achieved by addition of zirconium which precipitates iron from molten alloy. Desirably, the alloys contain a minimum of iron. Preferably, alloys of the present invention contain less than 0.005% iron and, most preferably, substantially no iron. Accordingly, the zirconium contents specified herein are residual zirconium contents. However, it is to be noted that zirconium may be incorporated at two different stages. Firstly, on manufacture of the alloy and secondly, following melting of the alloy just prior to casting. Preferably, the zirconium content will be the minimum amount required to achieve satisfactory iron removal. Typically, the zirconium content will be about 0.1% or less.

Manganese is an optional component of the alloy. When present, the manganese content will typically be about 0.1%.

Elements which prevent or at least inhibit oxidation of molten alloy, such as beryllium (Be) and calcium (Ca), are optional components which may be included especially in circumstances where adequate melt protection through cover gas atmosphere control is not possible. This is particularly the case when the casting process does not involve a closed system.

When present, the beryllium content is preferably less than 50 ppm, more preferably 4-25 ppm, more preferably 4-20 ppm, more preferably 4-15 ppm, more preferably 6-13 ppm, such as 8-12 ppm. Beryllium would typically be introduced by way of an aluminium-beryllium master alloy, such as Al-5% Be, and thus aluminium may be present in small amounts up to 0.25%. Preferably, the aluminium content is less than 0.2%, more preferably less than 0.1%. Without wishing to be bound by theory, the inclusion of beryllium and/or calcium is believed to improve the die castability of the alloy.

Ideally, the incidental impurity content is zero but it is to be appreciated that this is essentially impossible. Accordingly, it is preferred that the incidental impurity content is less than 0.15%, more preferably less than 0.1%, more preferably less than 0.01%, and still more preferably less than 0.001%.

Surprisingly for HPDC alloys, at least some alloys of the present invention can benefit from heat treatments, such as a T6 heat treatment which would typically involve solution treatment at 450-550° C. for up to 6 hours, followed by a quench, and then an artificial aging at 150-300° C. for up to 24 hours.

In a second aspect, the present invention provides a component of an internal combustion engine formed from an alloy according to the first aspect of the present invention. The component of the internal combustion engine may be the engine block or a portion thereof such as a shroud.

Alloys according to a first aspect of the present invention may be cast by processes other than HPDC, such as sand casting or low pressure/gravity permanent mould casting.

In a third aspect, the present invention provides an engine block for an internal combustion engine produced by high pressure die casting an alloy according to the first aspect of the present invention.

Specific reference is made above to engine blocks but it is to be noted that alloys of the present invention may find use in other elevated temperature applications such as may be found in automotive power trains as well as in low temperature applications. Specific reference is also made above to HPDC but it is to be noted that alloys of the present invention may be cast by techniques other than HPDC including thixomoulding, thixocasting, permanent moulding and sand casting.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION Example 1

Two alloys according to the present invention were prepared and chemical analyses of the alloys are set out in Table 1 below. The rare earths other than neodymium were added as a Ce-based misch metal which contained cerium, lanthanum and some neodymium. The extra neodymium and the zinc were added in their elemental forms. Standard melt handling procedures were used throughout preparation of the alloys.

TABLE 1 Alloys Prepared Element Alloy A Alloy B Nd (wt %) 0.6 0.54 Ce (wt %) 0.37 0.36 La (wt %) 0.32 0.31 Zn (wt %) 0.4 0.41 Y (wt %) 0.5 0.18 Mg (wt %) Balance Balance except for except for incidental incidental impurities impurities

The tensile properties of Alloys A and B at room temperature and at 177° C. are set out below in Table 2.

TABLE 2 Tensile Properties 21° C. 177° C. 0.2% Proof 0.2% Proof Stress UTS Elong. Stress UTS Elong. Alloy A 120 146 4.0 106 132 6.6 Alloy B 114 164 5.7 102 133 9.0

Creep tests were carried out on Alloys-A and B at a constant load of 90 MPa and at a temperature of 177° C. in the as-cast condition and in a T6 heat treated condition and at 75 MPa and 200° C. for Alloy A in the as-cast condition. The steady state creep rates are listed in Table 3.

TABLE 3 Steady State Creep Rates As Cast Steady State Creep Rates (s⁻¹) 90 Mpa 177° C. 75 Mpa 200° C. Alloy A 2.5 × 10⁻¹⁰ 4.3 × 10⁻¹⁰ Alloy B 3.0 × 10⁻¹⁰ —

FIG. 2 shows the creep results for 177° C. and 90 MPa for Alloys A and B in the as-cast condition. From FIG. 2 it can be seen that, although the two alloys have similar secondary creep rates, Alloy A is considerably more resistant than Alloy B to instantaneous strain upon loading under these conditions.

FIG. 3 shows the creep results for 177° C. and 90 MPa for Alloys A and B in a T6 heat treated condition. Alloys A and B were solution treated for 8 hours at 525° C., followed by a cold water quench, and then were aged at 215° C. for 4 hours. Under these conditions Alloy A is also considerably more creep resistant than Alloy B.

The influence of a T6 heat treatment on the creep behaviour of HPDC test specimens for Alloy A and Alloy B of the present invention is illustrated by a comparison of FIG. 2 with FIG. 3. It can be seen that a T6 heat treatment provides little advantage to low Y content alloys (<0.4 wt. % Y). However, for compositions that contain 0.5 wt. % Y or greater, a T6 heat treatment can have a significant beneficial influence on the creep performance of the alloy.

Example 2

A series of alloys according to the present invention were produced and their compositions are listed in Table 4 which includes Alloys A and B referred to in Example 1.

TABLE 4 Chemical composition of Alloys A-V Zr Zr Nd Ce La Y Zn Be Al Fe (wt. %, (wt. %, Alloy (wt. %) (wt. %) (wt. %) (wt. %) (wt. %) (ppm) (wt. %) (ppm) soluble) total) A 0.60 0.37 0.32 0.50 0.40 — — — — — B 0.54 0.36 0.31 0.18 0.41 — — — — — C 0.63 0.41 0.36 1.06 0.43 15 0.10 — <0.005 0.02 D 0.64 0.42 0.38 1.52 0.45 18 0.10 — 0.005 0.035 E 1.18 0.41 0.38 0.75 0.42 19 0.10 — 0.005 0.03 F 0.53 0.63 0.18 0.95 0.43 26 0.11 — 0.008 0.08 G 0.54 1.11 0.18 0.95 0.42 40 0.11 — 0.018 0.15 H 0.62 0.48 0.37 0.27 0.54 15 0.07 17 <0.005 <0.005 I 0.56 0.44 0.33 0.26 0.57 15 0.07 16 <0.005 <0.005 J 0.46 0.36 0.28 0.25 0.57 16 0.07 14 <0.005 <0.005 K 0.65 0.49 0.38 0.37 0.56 15 0.07 25 <0.005 0.009 L 0.58 0.44 0.34 0.39 0.57 18 0.08 27 0.005 0.010 M 0.49 0.37 0.28 0.40 0.54 17 0.08 20 0.005 0.010 N 0.67 0.50 0.37 0.54 0.58 16 0.07 26 <0.005 0.006 O 0.58 0.42 0.33 0.57 0.54 18 0.07 22 0.005 0.008 P 0.48 0.34 0.27 0.60 0.54 18 0.07 27 0.005 0.008 Q 0.48 0.35 0.28 0.64 0.80 20 0.07 34 0.006 0.010 R 0.46 0.34 0.27 0.61 1.12 19 0.07 28 0.005 0.010 S 0.68 0.51 0.39 1.17 0.57 <1 0.01 34 <0.005 <0.005 T 0.71 0.51 0.40 1.10 0.56 5 0.04 33 <0.005 <0.005 U 0.70 0.51 0.39 0.99 0.54 9 0.10 27 <0.005 <0.005 V 0.68 0.49 0.38 0.89 0.54 13 0.22 23 <0.005 <0.005

For the purposes of mechanical property evaluation, test specimens were produced by the high pressure die casting (HPDC) of these alloys on a 250 tonne Toshiba cold chamber machine. The alloy properties that were evaluated include casting quality, as-cast microstructure, tensile strength at room temperature and 177° C. and creep behaviour at 177° C. and 200° C.

A typical example of the microstructure of an alloy according to the present invention (Alloy N), in the as-cast condition, is shown in FIG. 4. Due to the nature of HPDC there is a transition from a fine grain structure, close to the surface of the cast specimen (the ‘skin’), to a coarser grain structure in the central region (the ‘core’). However, both regions consist of primary magnesium-rich grains or dendrites with a Mg-RE intermetallic phase in the inter-granular and interdendritic regions.

Alloys of the present invention are non-burning and highly resistant to oxidation as shown in FIG. 5.

FIGS. 5( d), 5(e) and 5(f) relate to an alloy of composition very similar to that of Alloy H and FIGS. 5( a), 5(b) and 5(c) relate to an alloy of equivalent composition save for it containing no yttrium. FIGS. 5( b) and 5(e) are macro images of polished sections through the centre of the castings shown in FIGS. 5( a) and 5(d) respectively which give an indication of the depth of penetration of the oxides that are formed on the surface into the interior of the castings. FIGS. 5( c) and 5(f) are equivalent higher magnification images of FIGS. 5( b) and 5(e) respectively. It can be seen in FIG. 5( c) that the yttrium free alloy displays extensive penetration of oxide stringers; whereas, there is minimal penetration evident in FIG. 5( f) of the alloy of the present invention.

The degree of surface oxidation and the depth of penetration of oxide stringers into the bulk of the casting are both greatly reduced for alloys with compositions typical of the present invention. This non-burning behaviour is very advantageous in all practical casting operations.

A summary of the tensile test data for the alloys of the present invention are given in Table 5 and it can be seen that the tensile behaviour is reasonable at both test temperatures considered. Examples of the stress-strain curves for Alloy N, Alloy 0 and Alloy P are shown in FIG. 6 and FIG. 7 for tests conducted at room temperature and 177° C. respectively. The influence of the Y content of the composition on the tensile stress-strain behaviour is shown in FIG. 8, where it can be seen that the tensile strength is improved with increasing Y content.

TABLE 5 Typical tensile properties for the range of example alloys of the present invention at both room temperature and 177° C. RT − 21° C. 177° C. 0.2% proof, 0.2% proof, UTS, Alloy (MPa) UTS, (MPa) % E (MPa) (MPa) % E A 120.1 ± 2.1 146.3 ± 2.8 4.0 ± 0.3 106.2 ± 2.7 131.6 ± 7.1 6.6 ± 1.0 B 114.0 ± 7.0  163.6 ± 11.2 5.7 ± 0.9 102.5 ± 2.9 133.4 ± 1.4 9.9 ± 0.6 C 122.4 ± 1.7 167.6 ± 4.6 5.2 ± 0.7 112.5 ± 0.5 150.2 ± 8.1 8.8 ± 1.6 D 127.5 ± 4.4 176.1 ± 5.9 5.6 ± 0.6 117.2 ± 1.6 151.7 ± 3.0 7.0 ± 0.3 E 128.6 ± 1.6  164.8 ± 12.6 4.5 ± 1.1 117.6 ± 1.5 146.0 ± 3.5 6.4 ± 0.9 F 120.7 ± 2.0 153.1 ± 6.2 4.3 ± 0.5 111.0 ± 1.8 137.0 ± 4.0 6.5 ± 0.8 G 130.7 ± 2.7 159.2 ± 7.2 4.2 ± 0.6 113.5 ± 1.9 142.1 ± 4.1 6.5 ± 0.9 H 118.1 ± 1.4 158.1 ± 4.7 4.6 ± 0.5 104.7 ± 1.6 131.8 ± 3.4 6.9 ± 0.2 I 115.0 ± 2.2  152.9 ± 11.6 4.5 ± 1.0  96.8 ± 1.9 130.4 ± 5.2 7.6 ± 0.8 J 110.5 ± 1.0 148.3 ± 7.5 4.5 ± 0.6  97.8 ± 3.0 127.5 ± 7.9 7.7 ± 1.8 K 126.3 ± 0.9 159.0 ± 6.1 4.65 ± 0.6  104.6 ± 6.9 130.7 ± 7.9 5.8 ± 1.1 L 120.6 ± 1.0 152.8 ± 4.8 4.2 ± 0.5 101.6 ± 5.5 126.7 ± 5.3 6.1 ± 0.7 M 114.5 ± 1.8 137.9 ± 2.1 3.34 ± 0.1  100.2 ± 1.9 123.5 ± 4.6 5.9 ± 0.3 N 127.3 ± 2.0  151.8 ± 11.1 3.67 ± 0.95 109.0 ± 3.5 137.7 ± 4.8 6.1 ± 0.3 O 126.1 ± 0.9 155.2 ± 1.7 4.1 ± 0.2 107.8 ± 2.1 133.0 ± 2.1 5.9 ± 0.9 P 116.8 ± 2.8 133.8 ± 4.2 2.7 ± 0.4 103.7 ± 2.4 120.3 ± 6.5 4.5 ± 0.9 Q 123.3 ± 1.3 150.8 ± 2.6 4.2 ± 0.3 105.8 ± 2.5 127.4 ± 4.9 5.2 ± 0.8 R 119.7 ± 2.2 147.9 ± 1.2 3.8 ± 0.3 102.9 ± 2.0 122.0 ± 4.0 4.8 ± 0.7 S 131.5 ± 4.5 159.8 ± 6.6 4.5 ± 0.5 121.1 ± 4.3 145.7 ± 2.3 5.2 ± 0.5 T 131.3 ± 3.7 155.3 ± 6.9 3.9 ± 0.7 118.9 ± 2.4 142.0 ± 4.7 5.0 ± 0.8 U 129.6 ± 4.0 151.1 ± 5.3 3.3 ± 0.5 114.6 ± 2.8 134.0 ± 9.0 4.6 ± 1.0 V 132.3 ± 4.2 155.3 ± 7.3 3.4 ± 0.7 115.1 ± 2.4 131.6 ± 6.6 3.9 ± 0.7

The steady-state creep rates for an alloy of the present invention (Alloy A, in the as-cast condition) under a number of different test conditions are contained in Table 6 and examples of the associated creep curves are also shown in FIG. 9.

TABLE 6 Steady-state creep rates for Alloy A of the present invention under various test conditions and under the same test condition for different heat treatment. Steady-state Creep Rates (s⁻¹) 90 MPa 177° C. As- 55 MPa 200° C. 75 MPa 200° C. Alloy cast T6 (As-cast) (As-cast) A 2.5 × 10⁻¹⁰ 3.6 × 10⁻¹¹ 1.1 × 10⁻¹⁰ 4.3 × 10⁻¹⁰

A summary of the steady-state creep rate under the same conditions of 177° C. and 90 MPa for all the composition variations measured in the as-cast condition is contained in Table 7.

TABLE 7 Steady-state creep rate for a number of alloy variants of the present invention. As-cast Steady-state Creep Rate (s⁻¹) Alloy 90 MPa 177° C. A 2.5 × 10⁻¹⁰ B 3.0 × 10⁻¹⁰ C 2.1 × 10⁻¹⁰ D 1.9 × 10⁻¹¹ E 3.4 × 10⁻¹⁰ F 3.0 × 10⁻¹⁰ G 2.6 × 10⁻¹⁰ H 1.3 × 10⁻⁹  I 6.3 × 10⁻¹⁰ J 4.6 × 10⁻⁸  K 2.6 × 10⁻¹⁰ L 2.0 × 10⁻¹⁰ M 2.5 × 10⁻¹⁰ N 1.4 × 10⁻¹⁰ O 9.0 × 10⁻¹¹ P 2.3 × 10⁻¹¹ Q 4.0 × 10⁻¹¹ R 1.6 × 10⁻¹⁰ S 4.2 × 10⁻¹⁰ T 1.7 × 10⁻¹⁰ U 3.1 × 10⁻¹⁰ V 2.0 × 10⁻¹⁰

Selected creep curves, for alloys with increasing Y content, are shown in FIG. 10, FIG. 11 and FIG. 12 for compositions that contain a low total rare earth (TRE) content (in the range 1.1-1.2 wt. %), a medium TRE content (in the range 1.3-1.4 wt. %) and a high TRE content (in the range 1.5-1.6 wt. %). It is a general observation that increasing the Y content of the alloys of the present invention results in a significant improvement to the creep behaviour observed under these test conditions (177° C. and 90 MPa). Alloy compositions with both a low TRE content and a low Y content display poorer creep performance under these stringent test conditions, as indicated by the curve for Alloy J in FIG. 10. Such compositions are therefore better suited to less demanding applications in the powertrain than the engine block. For compositions that contain in excess of 0.45 wt. % Y the creep performance is very good as shown by Alloy A, Alloy C and Alloy T (see FIG. 9, FIG. 11 and FIG. 12 respectively).

It is to be clearly understood that although prior art publications are referred to herein, this reference does not constitute an admission that any of these documents forms part of the common general knowledge in the art in Australia or in any other country. 

1. A magnesium-rare earth-yttrium-zinc alloy consisting of: rare earth(s) (RE) and yttrium in amounts which fall within a quadrangle defined by lines AB, BC, CD, and DA wherein: A is 1.8% RE-0.05% Y, B is 1.0% RE-0.05% Y, C is 0.2% RE-0.8% Y, and D is 1.8% RE-0.8% Y; 0.2-1.5% zinc; 0-0.25% aluminium; 0-0.2% zirconium; 0-0.3% manganese; 0-0.1% oxidation inhibiting element(s), and the balance being magnesium (Mg) except for incidental impurities.
 2. An alloy as claimed in claim 1 which contains rare earth(s) and yttrium in amounts which fall within a quadrangle defined by lined EF, FG, GH, and HE wherein: E is 1.5% RE-0.3% Y, F is 1.0% RE-0.3% Y, G is 1.0% RE-0.8% Y, and H is 1.5% RE-0.8% Y.
 3. An alloy as claimed in claim 1 containing at least 96.7% magnesium.
 4. An alloy as claimed in claim 1 wherein the rare earth element(s) are selected from neodymium, cerium, lanthanum, praseodymium, or any combination thereof.
 5. An alloy as claimed in claim 1 having a neodymium content of 0.4%-1.0%
 6. An alloy as claimed in claim 1 wherein the content of rare earth(s) other than neodymium is 0.5-1.0%.
 7. An alloy as claimed in claim 1 having a yttrium content of 0.1-1.6%.
 8. An alloy as claimed in claim 7 having a yttrium content of 0.25%-1.25%.
 9. An alloy as claimed in claim 8 having a yttrium content of 0.5-1.0%.
 10. An alloy as claimed in claim 1 having a zinc content of 0.2%-0.7%.
 11. An alloy as claimed in claim 10 having a zinc content of 0.4%-0.6%.
 12. An alloy as claimed in claim 1 containing aluminium in an amount less than 0.25%.
 13. An alloy as claimed in any claim 1 containing zirconium in an amount less than 0.2%.
 14. An alloy as claimed in claim 1 containing manganese in an amount less than 0.3%.
 15. An alloy as claimed in claim 1 containing beryllium in an amount less than 50 ppm.
 16. An alloy as claimed in claim 1 containing calcium in an amount less than 0.1%
 17. A component of an internal combustion engine or automotive power train formed from an alloy as claimed in claim
 1. 18. An engine block or portion thereof produced by high pressure die casting an alloy as claimed in claim
 1. 